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digitalmars.D.learn - Splitting a sequence using a binary predicate on adjacent elements

reply =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
I can't find any algorithm/range in Phobos that can be used to 
split (eagerly or lazily) a sequence using a binary predicate on 
adjacent elements as follows

[1,2,3,5,10,11,12,13,20,21,100].splitBy!"a + 1 != b"()

should evaluate to

[[1,2,3], [5], [10,11,12,13], [20,21], [100]]

.

Is there one?
Oct 17 2017
next sibling parent reply =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Tuesday, 17 October 2017 at 13:09:18 UTC, Nordlöw wrote:

 Is there one?


If not, what should we call it?...yet another overload of 
`splitter()` with a binary predicate and only a single parameter 
`Range input`?
Oct 17 2017
parent jmh530 <john.michael.hall gmail.com> writes:
On Tuesday, 17 October 2017 at 13:25:01 UTC, Nordlöw wrote:

 On Tuesday, 17 October 2017 at 13:09:18 UTC, Nordlöw wrote:

 Is there one?


 If not, what should we call it?...yet another overload of 
 `splitter()` with a binary predicate and only a single 
 parameter `Range input`?


Would probably need to use findAdjacent to get it to work
https://dlang.org/phobos/std_algorithm_searching.html#findAdjacent

So splitterAdjacent?
Oct 17 2017
prev sibling parent reply Andrea Fontana <nospam example.com> writes:
On Tuesday, 17 October 2017 at 13:09:18 UTC, Nordlöw wrote:

 I can't find any algorithm/range in Phobos that can be used to 
 split (eagerly or lazily) a sequence using a binary predicate 
 on adjacent elements as follows

 [1,2,3,5,10,11,12,13,20,21,100].splitBy!"a + 1 != b"()

 should evaluate to

 [[1,2,3], [5], [10,11,12,13], [20,21], [100]]

 .

 Is there one?


Try this:

import std.range;
import std.algorithm;
import std.stdio;

import std.typecons;

auto splitBy(alias F, R)(R range)
{
	
	auto tmp = range
		.map!(x => tuple(x, 0))
		.cumulativeFold!((a,b) => tuple(b[0], 
(!F(a[0],b[0]))?a[1]:a[1]+1))
		.chunkBy!((a,b) => a[1] == b[1])
		.map!(x => x.map!(y => y[0]));
	
	return tmp;
}

void main()
{
	[1,2,3,5,10,11,12,13,20,21,100].splitBy!((a,b) => a+1 != 
b)().writeln;
}


Andrea
Oct 17 2017
next sibling parent reply Andrea Fontana <nospam example.com> writes:
More phobos-ized version:

https://run.dlang.io/is/iwgeAl

Andrea
Oct 17 2017
parent =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Tuesday, 17 October 2017 at 15:47:25 UTC, Andrea Fontana wrote:

 More phobos-ized version:

 https://run.dlang.io/is/iwgeAl


Thanks!
Oct 17 2017
prev sibling parent reply =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Tuesday, 17 October 2017 at 14:15:02 UTC, Andrea Fontana wrote:

 auto splitBy(alias F, R)(R range)


Because of lazyness shouldn't it be named something with 
splitter, say splitterBy, instead?
Oct 17 2017
parent reply Andrea Fontana <nospam example.com> writes:
On Wednesday, 18 October 2017 at 06:45:37 UTC, Nordlöw wrote:

 On Tuesday, 17 October 2017 at 14:15:02 UTC, Andrea Fontana 
 wrote:

 auto splitBy(alias F, R)(R range)


 Because of lazyness shouldn't it be named something with 
 splitter, say splitterBy, instead?


Yes but I think it is something more similar to chunkBy, but 
chunkBy says that "predicate must be an equivalence relation, 
that is, it must be reflexive (pred(x,x) is always true), 
symmetric (pred(x,y) == pred(y,x)), and transitive (pred(x,y) && 
pred(y,z) implies pred(x,z)). If this is not the case, the range 
returned by chunkBy may assert at runtime or behave erratically."

If you try to use your data with chunkBy!"a != b+1", it does not 
work, as expected.

I think that my implementation could superseed the current one, 
since it seems to work in a more generic way.

Andrea
Oct 18 2017
parent reply =?UTF-8?B?Tm9yZGzDtnc=?= <per.nordlow gmail.com> writes:
On Wednesday, 18 October 2017 at 07:01:19 UTC, Andrea Fontana 
wrote:

 If you try to use your data with chunkBy!"a != b+1", it does 
 not work, as expected.


What's the motivation behind this limitation?

Without it

     chunkBy!"a + 1 == b"

is exactly what I want.
Oct 18 2017
parent Andrea Fontana <nospam example.com> writes:
On Wednesday, 18 October 2017 at 07:26:20 UTC, Nordlöw wrote:

 On Wednesday, 18 October 2017 at 07:01:19 UTC, Andrea Fontana 
 wrote:

 If you try to use your data with chunkBy!"a != b+1", it does 
 not work, as expected.


 What's the motivation behind this limitation?

 Without it

     chunkBy!"a + 1 == b"

 is exactly what I want.


Probably it's an implementation problem. On source code you read:

// Issue 13595
version(none) // This requires support for non-equivalence 
relations
 system unittest
{
     import std.algorithm.comparison : equal;
     auto r = [1, 2, 3, 4, 5, 6, 7, 8, 9].chunkBy!((x, y) => 
((x*y) % 3) == 0);
     assert(r.equal!equal([
         [1],
         [2, 3, 4],
         [5, 6, 7],
         [8, 9]
     ]));
}

And other.
Oct 18 2017